Fix: Support Hexadecimal HTML Entities in HtmlEntityMapper

[srvcti]

This PR fixes a long-standing known limitation (\TODO FIXME) in \HtmlEntityMapper.java\ where the parsing engine failed to recognize or map Hexadecimal HTML entities (e.g. A).

By introducing a non-capturing group to the Regex pattern and natively decoding the base-16 matcher group, Zimbra can now successfully identify, map, and defang hexadecimal unicode entities identically to decimal entities.

This dramatically improves webmail rendering fidelity and strengthens the anti-XSS defang engine against hexadecimal obfuscation bypasses.

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